In digital communications a digital signal receiver must recover the clock frequency of the digital signal transmitter from information contained within the transmitted digital signal. This process is often referred to as timing recovery. In many applications, timing recovery is accomplished by extracting a timing error from the statistical properties of the received digital signal. When the transmitted digital signal is subject to severe interference such as channel noise and multipath, algorithms that depend upon the properties of the transmitted digital signal may fail to recover the timing information.
To facilitate timing recovery, a known sequence referred to as a “training sequence” is often transmitted at regular intervals as a part of the main data stream. Information from the training sequence aids the digital signal receiver in the recovery of the transmitted data. In the presence of severe interference, the quality of performance of the timing error detector will depend upon the particular algorithm that is used to extract the timing information from the training sequence. Prior art algorithms extract the timing information from a single training sequence. That is, recovery of the timing information is done using only one training sequence transmission to recover timing error. Prior art algorithms that use only a single training sequence in the timing error recovery process may not perform well in the presence of severe signal interference.
There is therefore a need in the art for an apparatus and method that is capable of utilizing information from more than one training sequence to extract timing error information in order to more accurately recover a clock rate of a digital signal transmitter in the presence of severe signal interference.